The formation of the body pattern in embryogenesis is under control of prelocalized cytoplasmic determinants which trigger differential gene expression in a spatial order. In the chiromid midge Smittia an embryonic malformation can be experimentally induced in 100% of treated eggs. In the malformed embryos head and thorax are replaced by a second abdomen joined in mirror image symmetry to the normal abdomen. The switch from head and thorax to abdomen formation apparently results from inactivation of cytoplasmic ribonucleoprotein (RNP) particles acting as "anterior determinants". Three groups of experiments are proposed. (1) The localization of anterior determinants apparently changes during nuclear migration. This will be analyzed by the kinetics of double abdomen yields after UV irradiation with different egg orientations at successive stages of development. These changes will be correlated with changes in the egg architecture and in the localization of radioactive poly(U)-probes hybridized in situ to egg sections. (2) Eggs programmed to develop the abnormal body pattern will be used to test the ability of transplanted fractions to reprogram ("rescue") eggs to form the normal pattern. With this bioassay, we will try to identify positively the molecular nature of the anterior determinants and their origin during oogenesis. (3) Indicator proteins that signal the development of the malformation before it becomes morphologically apparent will be analyzed by two-dimensional gel electrophoresis. We will compare the proteins synthesized in anterior versus posterior egg halves, both in normal and UV-irradiated eggs. Using inhibitors of transcription, we will discriminate between maternal and embryonic mRNAs coding for indicator proteins.